1. Field of Art
This invention relates to electrostatic charge sensors, and more particularly to use of asymmetric high voltages on corona current sensing probes.
2. Description of the Prior Art
All bodies isolated in the atmosphere, such as aircraft, are subject to electrostatic charging due primarily to induction from the earth's field and triboelectric charging due to contact with dust, rain and charged particles in the air; additionally, aircraft are subject to ionic charging from the exhaust gases of the engines. Considering the fact that the earth's electric field may vary from 100 to 1500 volts per meter near the earth's surface (although it varies considerably and may reverse its direction and/or sign locally) and that potentials as much as 360,000 volts at 50 kilometers altitude are not unusual, it is not surprising that charge voltages on helicopters have been measured of the order of 20,000 to 50,000 volts, and in at least one case as high as 200,000 volts. Discharges between masses such as helicopter and cargo, or from masses to ground, create an undesirable possibility of igniting fuels or explosive cargoes. And, there is significant danger to earth-grounded humans, either in the handling of cargo suspended from an electrically-charged aircraft or in rescue operations.
To overcome this, it is known to measure the electrostatic charge accumulated on aircraft and other bodies, to provide a visible indication thereof and/or to neutralize the charge by means of high voltage power supplies. Typical apparatus for sensing and neutralizing electrostatic charge is described in U.S. Pat. No. 3,857,066. That system provides for a zero balance bias adjustment so that the sensing circuitry associated with the probe will provide zero output for zero electrostatic field being sensed.
The problem with systems of the general type disclosed in the aforementioned patent is that the indication of magnitude of electrostatic field is erratic with changes in atmospheric conditions in which electrostatic charge is being sensed. For instance, if the device is accurate in still air, there may be a considerable error (even 50% or more) in a high wind. Variations in water content and temperature also affect the response. And, it can be shown that no point can be found where a zero output is achieved for zero electrostatic field across all of the normal atmospheric conditions in which sensing of electrostatic charge on aircraft must be carried out.